1. Field
This disclosure relates generally to power transistor feedback circuits and, more specifically, to power transistor feedback circuits that have noise and offset compensation.
2. Related Art
Power transistors typically have driver circuits that use load information for providing the desired control in driving the load. This is particularly common in driver circuits for motor control applications. In motor control applications the environment can have significant noise which can be present on the power supply, which may be a battery. In such environments the noise may be sufficient to adversely impact the performance of the circuits including the motor control circuit itself such as the feedback portion of the circuit that provides information concerning how the load is being driven.
Shown in FIG. 1 is a power circuit 10, according to the prior art, comprising a driver 12, an N channel transistor 14, an N channel transistor 16, an N channel transistor 20, a feedback amplifier 18, and a load 22. The driver 12 provides an activating signal to transistor 16 to cause transistor 16 to provide current to load 22 at an output of the power circuit. The activating signal is also received by transistor 14. Feedback amplifier 18 has a non-inverting input coupled to the output of power circuit 10, an inverting input coupled to a source of transistor 14, and an output coupled to a gate of transistor 20. During proper operation, feedback amplifier 18 senses the voltage on load 22 with the result that the non-inverting input is also at the voltage on load 22. With transistors 14 and 16 having the same drain, gate, and source voltages, the current through transistor 14 is a known ratio of the current through transistor 16 based on the channel widths and lengths of transistor 14 and 16. Thus the current through transistor 20 is a measure of the current through load 22. Thus, the bias conditions for transistor 20 are useful for driver 12 in determining if the current provided to load 22 is providing desired drive. A problem may arise with the operation of amplifier 18 when the power supply has large amounts of noise. These large amounts are sufficiently common that there is a standard test called direct power injection (DPI) that is used to determine if power circuit 10 can properly function under the high noise conditions. In this test, feedback amplifier 18 may not be able to provide proper function unless the circuit is designed to include noise compensation. This drawing is somewhat simplified for clarity. For example, there is commonly transistors that are added to ensure that if the battery is reversed these transistors become non-conductive and prevent damage to the circuitry that may otherwise occur if the battery terminals were reversed.
Shown in FIG. 2 is feedback amplifier 18, according to the prior art, in more detail and having noise compensation. The inverting input is shown as (−), the non-inverting input is shown as (+), and the output is connected to the gate of transistor 20 and driver 12. A standard amplifier is made up of PNP transistors 30 and 42, P channel transistors 32 and 38, and current sources 34 and 48. Transistors 30 42 function as one current mirror and transistors 32 and 38 function as another current mirror. Capacitor 36 is a parasitic capacitance that contributes to problems caused by the high noise conditions Transistor 50 and current source 52 are common for providing the actual output. NPN transistors 28 and 44, resistors 26 and 46, and capacitor 47 are useful in providing noise compensation which keeps feedback amplifier 18 functioning properly in high noise conditions. A disadvantage though is that there is an increase in the offset error, which is the difference voltage for the inverting and non-inverting voltages during the operation of the kind shown in FIG. 1. This results in the bias conditions on transistors 14 and 16 being different so that the current ratio is not as precisely known as without the noise compensation. The effect is that information received by the driver is less accurate resulting in the current through the load is less precisely controlled.
Accordingly there is a need for a feedback amplifier that improves upon the issues raised above.